Pressure loss is one of the important parameters in hydraulic engineering. Especially for the large Reynolds Number, relative roughness has a significant effect on pressure losses in annulus. In this study, water flowing through rough and smooth concentric annulus for turbulent flow is modeled using Computational Fluid Dynamics (CFD). CFD software based on the finite element method predictions of pressure losses are compared with experimental data obtained from the literature. The model predicts measured frictional pressure gradient with an error less than ±15% in all cases. Furthermore, pipe roughness influences on frictional pressure losses of water are also examined for various pipe roughness values, inner and outer diameters and Reynolds Number.  The analysis results revealed that as the diameter ratio increases, the influence of roughness on pressure gradient increases.